The orientational ordering of liquid crystals (LCs) is exceptionally sensitive to the structures and properties of surfaces or interfaces with which they are in contact. Because changes in the orientation of LCs can propagate over large distances (e.g., up to tens of micrometers) through the bulk, changes in the properties of LC interfaces that lead to small perturbations in orientation can be amplified and observed readily using polarized-light. This aspect of LC-based systems has been exploited to develop new sensing platforms that can report on the presence and/or organization of chemical or biological agents.
For example, several recent studies have demonstrated that ordering transitions in LCs can be triggered by the adsorption of phospholipids (Abbott, N. L. J. Am. Chem. Soc. 2008, 130, 4326; Brake, J. M.; Abbott, N. L. Langmuir 2007, 23, 8497), surfactants (Brake, J. M.; Mezera, A. D.; Abbott, N. L. Langmuir 2003, 19, 6436), polymers (Price, A. D.; Schwartz, D. K. J. Am. Chem. Soc. 2008, 130, 8188; Kinsinger, M. I.; Buck, M. E.; Campos, F.; Lynn, D. M.; Abbott, N. L. Langmuir 2008, 24, 13231), proteins (Park, J. S.; Abbott, N. L. Adv. Mater. 2008, 20, 1185), and viruses and bacteria (Sivakumar, S.; Wark, K. L.; Gupta, J. K.; Abbott, N. L.; Caruso, F. Adv. Funct. Mater. 2009, 19, 2260) at interfaces created between LCs and solid substrates or LCs and aqueous phases. The results of these past studies have suggested new principles and approaches for the design of LC-based systems that have relevance in a broad range of fundamental and applied contexts (e.g., sensing).
Past studies on the behavior of aqueous/LC interfaces have focused, in large measure, on the design and investigation of interfaces that are approximately planar (for example, interfaces formed by creating a thin film of a thermotropic LC between a solid substrate and a bulk aqueous phase) (Lockwood, N. A.; Gupta, J. K.; Abbott, N. L. Surf. Sci. Rep. 2008, 63, 255). In these experimental systems, planar solid substrates are used to provide a physical support for thin films of LC and as a means to define or control the orientation of the LC at one boundary.
It has been shown that amphiphilic polymers can assemble at planar aqueous/LC interfaces in ways that trigger ordering transitions in the LCs, and studies have also demonstrated that it is possible to design polymers and polymer-decorated LC interfaces that respond to external stimuli (e.g., changes in the pH of the aqueous phase or the presence of oppositely-charged polyelectrolytes) (Kinsinger, M. I.; Buck, M. E.; Meli, M. V.; Abbott, N. L.; Lynn, D. M. J. Colloid Interface Sci. 2010, 341, 124).
An alternative geometry that has recently been explored involves the use of LC droplets dispersed in a continuous aqueous phase (Gupta, J. K.; Zimmerman, J. S.; de Pablo, J. J.; Caruso, F.; Abbott, N. L. Langmuir 2009, 25, 9016). This approach has the advantage that surface treatment of solid substrates is not required to define the orientation of the LC. Several past studies have demonstrated that the interfaces of LC emulsion droplets can be decorated by the spontaneous adsorption of surfactants or phospholipids, and that these interfaces can be tailored to drive ordering transitions involving topological defects that are induced by the spherical geometries (Gupta, J. K.; Sivakumar, S.; Caruso, F.; Abbott, N. L. Angew. Chem., Int. Ed. 2009, 48, 1652). In addition to eliminating the need for solid substrates, as noted above, the confinement of the LC into spherical geometries offers new approaches to manipulate the ordering of the LCs (Drzaic, P. S., Liquid Crystal Dispersions. World Scientific Publishing Co.: Singapore, 1995).
For example, the ordering of LCs confined within droplets is sensitive to the size of the droplets, thereby providing additional means to tune the response of the LC (e.g., ordering transitions) to interfacial events by controlling droplet size (Gupta, J. K.; Sivakumar, S.; Caruso, F.; Abbott, N. L. Angew. Chem., Int. Ed. 2009, 48, 1652). In this context, past studies have described methods to encapsulate LC droplets dispersed in aqueous media based on the assembly of water-soluble polymers at the interfaces of the droplets (Sivakumar, S.; Gupta, J. K.; Abbott, N. L.; Caruso, F. Chem. Mater. 2008, 20(6), 2063-2065).
However, in the case of non-planar LC aqueous interfaces such as those characteristic of LC dispersions, a complicating factor is the ability of the droplets to move freely within the surrounding medium. While a high level of droplet mobility can be desirable in certain contexts, it can also create substantial challenges with respect to the characterization of individual LC droplets and any functions associated with the characterization of such droplets, such as droplet-based sensing.